It is known that lift trucks, hand trucks, and the like may perform lifting and transporting functions. Weighing functions may also be carried out with this type of equipment when fitted with a weight measuring system. However, specific problems can and do arise with prior art systems with regard to carrying out lifting and weighing operations, manufacturing, installation and servicing. For example, some prior art systems require structural modifications or additional attachments to the lift truck, adding cost and resulting in more difficult and time consuming installation. See U.S. Pat. Nos. 4,421,186 and 6,002,090.
Some prior art systems are time consuming and cumbersome to operate because the device must be set to a first configuration in order to perform weighing operations and a second configuration in order to perform transport operations. This requires the operator to perform additional steps before weighing functions can be performed. See U.S. Pat. No. 5,739,478.
Some prior art systems employ technically and physically elaborate approaches to address excessive vertical forces, lateral forces and binding in the weighing system. Excessive forces and binding can cause inaccurate weight readings and damage to the load cells. See U.S. Pat. Nos. 4,421,186 and 6,002,090. See also the inventor's previous patent, U.S. Pat. No. 6,730,861 (“the '861 patent”) wherein there is claimed a lift truck fork apparatus which includes two load cells, both of which are attached to the cover of the apparatus. A fixed linkage between the cover and the load cells ought to create a more direct, and thus accurate, measurement system. However, in practice, irregularities in the geometry of typical forks create an imperfect base upon which a load cell is mounted. When a load is applied, the imperfect fork may twist slightly, thereby resulting in the creation of a shear force. This results in the load cells inaccurately measuring the weight of the load. A further problem with the design described in the '861 patent is that the shear forces may exceed the limit of the bolts which attach the load cell to the cover, resulting in failure of these bolts.
Some prior art systems use only a portion of the lifting surface for weighing, which limits load placement options, and employ partial shrouds or covers that do not extend over the entire fork and subassembly surface. Such systems are prone to false weight readings due to contamination of the weighing subsystem by foreign substances such as dirt and water. See U.S. Pat. No. 4,420,053.
Some prior art systems employ mechanically elaborate designs to address weight measurement accuracy problems resulting from eccentric loads. These are likely to be prone to mechanical malfunction and high manufacturing cost. See U.S. Pat. No. 4,368,876.
Some prior art system designs are not based on standard lift truck fork configurations and are much thicker or much higher in cross section, or have component details that protrude far above the general height of the fork lifting surface, thereby making it more difficult or impossible to slide the forks under a standard pallet. See U.S. Pat. Nos. 4,899,840 and 5,861,580. Further, prior art systems not based on standard lift truck forks tend to require more custom manufactured components and complexity, usually resulting in higher manufacturing costs and requiring more highly skilled or knowledgeable service personnel.
Some prior art systems require additional weigh system attachment components, such as a secondary carriage, that can create a potentially unbalanced and unsafe condition because the position of the forks is moved forward relative to the lift truck, thereby resulting in incorrect load centers specified by the lift truck manufacturer. See U.S. Pat. Nos. 4,421,186 and 6,002,090.
Accordingly, there is a need for a weighing device which is fast, easy, and safe to use; simple to build and install; and is resistant to inaccuracy due to lateral forces or contamination by foreign substances.